Field of the Invention
The present invention relates to a liquid crystal drive apparatus configured to drive a liquid crystal element by a digital driving method.
Description of the Related Art
Liquid crystal elements include transmissive liquid crystal elements such as a TN (Twisted Nematic) element and reflective liquid crystal elements such as a VAN (Vertical Alignment Nematic) element. These liquid crystal elements are driven by an analog drive method and a digital drive method. The analog drive method changes a voltage applied to a liquid crystal layer depending on tones to control lightness (brightness), and the digital drive method binarizes the voltage applied to the liquid crystal layer and changes a voltage application time period to control lightness. As such a digital drive method, a sub-frame drive method temporally divides one frame period into multiple sub-frame periods and controls application (ON) and non-application (OFF) of a predetermined voltage to each pixel to cause the pixel to display its tone.
Description will be made of a typical sub-frame drive method. FIG. 15 illustrates an example of dividing one frame period into multiple sub-frame periods (bit lengths). Numerical values written in the respective sub-frames indicate temporal weights of these sub-frames in the one frame period.
The example shows a case of expressing 64 tones. In this example, a sub-frame period having a temporal weight of 1+2+4+8+16 is referred to as “an A sub-frame period”, and a sub-frame period having a temporal weight of 32 is referred to as “a B sub-frame period”. Furthermore, a sub-frame period where the predetermined voltage is applied is referred to as “an ON period”, and a sub-frame period where the predetermined voltage is not applied is referred to as “an OFF period”.
FIG. 16 illustrates all tone data corresponding to the division example illustrated in FIG. 15. A vertical axis indicates tones, and a horizontal axis indicates one frame period. A white sub-frame period indicates the ON period where the pixel is in a white display state, and a black sub-frame period indicates the OFF period where the pixel is in a black display state. According to these tone data, when two pixels adjacent to each other (hereinafter referred to as “adjacent pixels”) in a liquid crystal element display two tones adjacent to each other (hereinafter referred to as “adjacent tones”) such as 32 and 33 tones, the 32 tone is displayed by setting the A sub-frame period to the ON period and setting the B sub-frame period to the OFF period, and the 33 tone is displayed by setting the A sub-frame period to the OFF period and setting the B sub-frame period to the ON period.
Such a state where the ON and OFF periods temporally overlap each other in the adjacent pixels, that is, the predetermined voltage is applied to one (ON-period pixel) of the adjacent pixels and the predetermined voltage is not applied to the other one (OFF-period pixel) of the adjacent pixels generates so-called disclination, which generates a decrease in lightness of the ON-period pixel. FIG. 17 illustrates an example of the decrease in lightness due to the disclination. FIG. 19 illustrates tones in its vertical direction, and its contrasting density illustrates displayed lightness. When the disclination is not generated, a smooth contrasting density can be expressed. However, when the adjacent pixels display two adjacent tones (such as the 32 and 33 tones) corresponding to a case where the ON and OFF periods overlap each other for a long time, the displayed lightness is decreased due to the disclination, which generates a dark line.
Japanese Patent Laid-Open No. 2013-050681 discloses a drive circuit that divides one or more long sub-frame periods into periods each equal to a short sub-frame period to produce multiple divided sub-frame periods. The drive circuit disclosed in Japanese Patent Laid-Open No. 2013-050681 performs, when phases of bits of tone data corresponding to adjacent pixels are mutually different, a process to maintain their tones and corrects a bit arrangement of the tone data corresponding to one of the adjacent pixels so as to make it closer to a bit arrangement of the tone data corresponding to the other one of the adjacent pixels. This process enables, compared with a case of not dividing the long sub-frame period, shortening the sub-frame period (hereinafter referred to as “an ON/OFF adjacent period”) where the ON and OFF periods mutually overlap between the adjacent pixels.
Furthermore, some configurations of the tone data cause a false contour in a displayed motion image. Japanese Patent Laid-Open No. 2013-050682 discloses, as illustrated in FIG. 18A, a drive circuit that divides one frame period into multiple sub-frame periods each corresponding to each bit of the tone data and having a period depending on a weight of the corresponding bit.
The drive circuit disclosed in Japanese Patent Laid-Open No. 2013-050682 further rearranges, as illustrated in FIG. 18B, part of the multiple divided sub-frame periods, which are produced by dividing the one or more long sub-frame periods into the periods each equal to the short sub-frame period, to sub-frame periods different from those before the division in the one frame period. Such a drive circuit enables, since dividing the long sub-frame period into the periods each equal to the short sub-frame period, reducing a generation of a white and black boundary that is generated due to a small difference of tones and exits for a long time, which enables making it difficult that the false contour is generated. FIG. 18C illustrates tone data disclosed in Japanese Patent Laid-Open No. 2013-050682. These tone data includes, using data of 1 bit as a unit, nine data whose ratio of periods is 4:4:4:4:1:2:4:4:4, and combining these nine data enables expressing 32 tones.
However, in the method disclosed in Japanese Patent Laid-Open No. 2013-050681, a shortest ON/OFF adjacent period of the adjacent pixels is too long to ignore the decrease in lightness due to the disclination. Furthermore, in the method, a long ON/OFF adjacent period of the adjacent pixels increases an amount of the decrease in lightness due to the disclination depending on a response speed of liquid crystal molecules.
FIG. 19 illustrates all tone data disclosed in Japanese Patent Laid-Open No. 2013-050681 where an A sub-frame corresponds to a temporal weight of 1+2+4+8 and a B sub-frame is divided into multiple divided sub-frame periods 1SF (SF means a sub-frame) to 10SF each corresponding to a temporal weight of 8. One divided sub-frame period is 0.69 ms. In the tone data, the shortest ON/OFF adjacent period of the adjacent pixels is 1.39 ms that corresponds to two divided sub-frame period. Thus, the decrease in lightness (that is, the dark line) due to the disclination is noticeable.
Furthermore, the drive circuit disclosed in Japanese Patent Laid-Open No. 2013-050682 can reduce the generation of the false contour in the displayed motion image, but cannot reduce a generation of a multiple image. For example, FIG. 20 illustrates a multiple image generated when a single line of 15 tone in the tone data illustrated in FIG. 18C is displayed and horizontally scrolled (moved) in a black background display. To display the 15 tone, a white display in the white display state and a black display in the black display state are switched in the following temporal order: the black display in SF5-1; the white display in SF4-1; the black display in SF5-2; the white display in SF3, SF1 and SF2; the black display in SF5-3; the white display in SF4-2; and the black display in SF5-4. As just described, the white display is intermittently performed three times in the one frame period.
FIG. 20 illustrates, at its left part, frame images switched every 60 Hz and each displaying a white line horizontally scrolled in a black background three times. A vertical axis indicates time, and a horizontal axis indicates display coordinates. In addition, FIG. 20 illustrates, at its right part, display times (vertical axis) of the white line and the display coordinates (horizontal axis) using pulse waveforms. A size of each pulse waveform indicates a relative lightness of the white line with respect to the black background.
In FIG. 20, in a first frame where a left upper frame image is displayed in 60 Hz, the white line of the 15 tone is displayed three times.
In a second frame where a next middle frame image is displayed in 60 Hz, the white line is also displayed three times and its display coordinate is moved by the scrolling. In a third frame where a further next lower frame image is displayed in 60 Hz, the white line is also displayed three times and its display coordinate is further moved by the scrolling. As just described, the white line scrolled during the three frames is displayed three times in each frame. A viewer pursues, because of a pursuit characteristic of human's eyes, the three white lines temporally separated as indicated by arrows and thereby visually recognizes a triple line (multiple image). Moreover, when the 15 tones and 16 tones adjacent thereto are displayed mutually adjacent pixels, the white and black displays are performed for a long period, which generates the disclination.
Accordingly, it is necessary to set the tone data capable of reducing the generation of the disclination due to the adjacent tones and avoiding the visual recognition of the multiple image in motion image display.